A 51-year-old male has just been transferred to ICU from the surgical ward with worsening shortness of breath five days post-oesophagectomy, and a presumed anastomotic leak.
On arrival in ICU he is tachypnoeic and extremely agitated.
Arterial blood gas analysis on FiO2 0.6-0.8 via reservoir (non-rebreathing) mask shows:
Parameter | Patient Value | Normal Adult Range |
pH | 7.12* | 7.35 – 7.45 |
PaO2 | 50 mmHg (6.6 kPa) | |
PaCO2 | 50 mmHg (6.6 kPa)* | 35 – 45 (4.6 – 6.0) |
HCO3 | 16 mmol/L* | 22 – 28 |
Chest X-ray shows bilateral pulmonary infiltrates.
a) List the possible causes for his respiratory failure.
The patient is intubated and mechanical ventilatory support is initiated.
b) Describe the ventilator settings you will prescribe, giving the rationale for your decision.
Following intubation, there is no immediate improvement in the patient’s oxygenation
c) List the initial strategies that may be used to improve oxygenation.
College Answer
a) List the possible causes for his respiratory failure.
Differential diagnosis should include:
ARDS secondary to sepsis from any source or other inflammatory insult including the following
Pneumonia (hospital-acquired)
Aspiration
Atelectasis/pleural effusions/empyema
Fluid overload secondary to resuscitation, renal failure
Exacerbation of pre-existing condition e.g. heart failure, valvular heart disease, post-op
ischaemia/MI, arrhythmia
Lung diseases e.g. lymphangitis carcinomatosis
The patient is intubated and mechanical ventilatory support is initiated.
b) Describe the ventilator settings you will prescribe, giving the rationale for your decision.
Use a mode with which one is familiar and aim to limit ventilator-associated lung injury, i.e
oxygen toxicity, barotrauma, volutrauma, shear stress and biotrauma
Choice of mode (any appropriate answer acceptable e.g. APRV for recruitment benefit, or
volume assist control as staff familiarity and no one mode shown to have benefit over another)
Avoid over-distention of alveoli by keeping tidal volumes at 6-8 ml/kg (predicted body weight
which in the ARDSnet studies was ~20% below actual body weight and calculated by a formula
linking height and sex)
Use PEEP to minimise alveolar collapse and derecruitment.
Titrate PEEP to achieve a PaO2 of 60 mmHg with lowest FiO2 that is needed using decremental
PEEP trial post recruitment manoeuvre.
I:E ratio of 1:1
Permissive hypercapnea to avoid large minute volumes and alveolar injury through collapse and expansion of lung units
Following intubation, there is no immediate improvement in the patient’s oxygenation
c) List the initial strategies that may be used to improve oxygenation.
High FiO2 (titrated to lowest possible level to limit toxicity)
Confirm ETT position and patency
Exclude readily reversible cause of hypoxia e.g. PTX, mucus plug, large effusion
Increased inspiratory time
Increased PEEP
Recruitment manoeuvre with decremental PEEP trial
Prone positioning for at least 16/24 hours per day
Ensure adequate cardiac output
Discussion
a) List the possible causes for his respiratory failure.
In the chapter on the definition, causes and differential diagnosis of ARDS there is this table:
Vascular:
Infectious
Neoplastic
Idiopathic
|
Drug-induced
Autoimmune
Traumatic
|
The next question asks the candidate to describe the ventilator settings you will prescribe, giving the rationale for your decision. After those settings fail to work, the college asks for strategies that may be used to improve oxygenation. Overall this question is asking about Ventilation strategies for ARDS. These are discussed in greater detail elsewhere. A brief summary may be offered:
Initial ventilator strategy:
- Use a Pressure Control mode (it may be safer, though the evidence is not strong)
- Lung-protective ventilation: use low tidal volumes (6ml/kg)
- Open-lung ventilation: avoid derecruitment by using a high PEEP
- The ideal PEEP can be found either by finding the lower inflection point or the pressure-volume curve or by observing a stepwise decrease in PEEP after a recruitment manoeuvre.
- As the ARDS severity increases, consider using a higher PEEP.
- Use a lower driving pressure (ΔP) -Amato et al, 2015. That means, using a higher PEEP and aiming for a lower plateau pressure
- Accept a level of "permissive hypercapnea"
Additional ventilator manoeuvres to improve oxygenation:
- Use an I:E ratio of 1:1, even though manipulating the I:E ratio does not seem to improve survival, even though it may improve oxygenation.
- One might attempt some recruitment manoeuvres if hemodynamics permit. Again, these offer a transient improvement in oxygenation, but do not influence survival.
Non-ventilator adjunctive therapies for ARDS:
- Minimization of dead space ventilation - Remove as much tubing as you can.
- Low-carbohydrate high-fat nutrition - Keep them off the carbs, and don't overfeed.
- Neuromuscular blockade improves survival, not just gas exchange.
- Sedation decreases energy expenditure and improves ventilator synchrony
- Fluid management should have a goal of neutral balance (keep em dry)
Ventilator strategies to manage refractory hypoxia
- Prone ventilation, for at least 16 hours a day (PROSEVA, 2013)
- High frequency oscillatory ventilation may not improve mortality among all-comers (OSCAR, 2013) or it may actually increase mortality (OSCILLATE, 2013) but some authors feel that there were problems with methodology.
Non-ventilator adjuncts to manage refractory hypoxia
- Nitric oxide was a cause for some excitement, but is no longer recommended.
- Prostacyclin is still a cause for excitement, and is still vaguely recommended.
- Neither agent improves mortality, but prostacyclin can improve oxygenation.
- ECMO may improve survival (CESAR, 2009) but again there were problems with methodology.
References
Blanco, Silvia, and Antoni Torres. "Differential Diagnosis of Pulmonary Infiltrates in ICU Patients." www.antimicrobe.org
ARDS Definition Task Force. "Acute Respiratory Distress Syndrome." Jama307.23 (2012): 2526-2533.